WINDMILL is a mill for grinding corn, sawing wood, or performing any other species of work for which fixed machinery can be employed, the motive-power being the force of the wind acting on a set of sails in a manner similar to that of a current of water impinging obliquely on the float-boards of a water-wheel. The structure is a con ical or pyramidal tower of considerable height, and covered over at the top with a species of dome, aaa (fig. 1), which is so fastened as to revolve upon it round the upper extrem ity of the shaft c, as a center, the motion being aided by the interposition of "castors" between the wooden rings which form respectively the base of the dome and the top of the tower; the sails, b, 1 are attached to the extremity of the axis d, so as to revolve in a plane at right angles to it, and the motion they communicate to the axis is transferred by the beveled wheels e and f to the upright shaft c, by which it is in turn conveyed to the work ing machinery at the bottom of the tower.
The axis d of the sails, which is inclined at an angle of about 10° to the horizontal, is fixed at one end to a projection from the top of the shaft c, and at the other to a circular orifice in the side of the dome,, so that it revolves with the latter, car rying the sails along with it; this arrange went is adopted for the purpose of enabling the plane of rotation of the sails to be placed always at right angles to the direction of the wind. This transference of the plane of rotation was at one time effected by manual labor applied to a winch at the bottom of the tower, the rotation being communicated, by an endless band and wheel-work above, to the dome, The outer circumference of the base of which, was, for this purpose, fur nished with a circle of rack-work. But this clumsy arrangement was superseded in English windmills by an ingenious contrivance due to sir William Cubitt, by which the wind itself was made to turn the sails into their proper position. The apparatus by which this is effected consists of a revolving flyer or fan, g, projecting from a gallery fastened to the dome on the side opposite to the sails; h, a long thin shaft to which a revolving motion is communicated by a toothed wheel on its outer extremity, from a corresponding wheel on the axis of the flyer (these wheels are not seen in the fig., being behind the flyer); a pinion at the other end of the shaft acts upon the cog-wheel /:,, which carries, on the lower extremity of its axis, a pinion 1; and this last can, at pleasure, be put into gearing with the rack-work or cog-circle on the lower edge of the dome. The sails are four in number. Each sail consists of a whip or radius of from 33 to 40 ft. in length, firmly fastened at right angles to the sail-axle, and pierced at from I- or of its length, from the axle to its extremity with about 20 holes, into each of which is inserted a cross bar of 5 to 6 ft. in length; and this frame-work, strengthened generally by light rods connecting the ends of the cross-bars, is then covered with canvas. The
cross-bars, however, are not set in the plane of revolution of the whips, for, in that case, the wind, acting in a direction coinciding with that of the sall-axle, would impinge per pendicularly on the sails, and no rotatory motion would result; the bars, therefore, arc set at an angle to this perpendicular direction, yet not all at the same angle, for the velocity of each point of the sail increasing with its distance from the sail-axle, the inclination must vary from the first cross-bar to the outer extremity. It is found that a variation of the angle from 18°, at the first cross-bar, to 7° at the extremity, is a very effective form. The amount of sail that a windmill can carry with advantage is limited, according to Mr. Smeaton (q.v.), one of the great authorities on this subject, to I of the area of the circle described by one whip; the velocities of a sail, when unconnected with, and when producing its maximum effect on the machinery below, are as 3 to 2; also, the increase of useful effect varies with the square of the wind's velocity, and is propor tional to the cube of the length of the whip, in sails of similar form. A windmill with sails of 40 ft. radius is equivalent to 65,000 foot-pounds per minute. Another species of windmill, known as a horizontal windmill, is a large circular frame of wood which rotates on a vertical axis, and carries a set of sails which revolve in a horizontal plane. This form is, however, not nearly so effective as the other, it being evi dent that the wind can only act effectively on one sail at a time. According to sir David Brewster, the power of a horizontal mill is only about one-third or one-fourth of that of a vertical mill, the number and size of the sails being equal in each. An ingeni ous form of horizontal windmill was patented by Mr. Giraudat of New York in 1861. The peculiarity is in the sails, which are hinged in such a way that the force of the wind acting on one face of them preserves their perpendicularity to it, and secures a maximum effect, but when, after a further semi-revolution, the other side is presented to the wind, they are raised to a horizontal position. Most of the le improvements mprovements in windmills have had for their object the regulation of the sail-area exposed to the wind to counterbalance the variations in the latter's force, and so produce uniformity of motion; but these are too numerous to be here noticed. 1 ed. We may mention, however, ;hat the inventions, with this object, of Mr. l3ywater in England, M. Berton in. Frace, and Mr. Henry Glover of Massachusetts, are both ingenious and effective. Windmills were introduced into Europe from the Saracens, and were formerly much more exten sively used in England than now. They are, however, still common in the midland and southern districts; on the continent, especially in Holland and France; and in the United States.